Following our success in developing a photoaffinity-labelled tetrodotoxin analog for identification of the tetrodotoxin receptor in the outer mouth of the sodium pore (Guillory et al., Science 196:883, 1977), we seek to extend this approach to other pharmacological agents with similarly specific binding sites within and around the pore. Two major classes of agents will be developed using the arylazido-beta-alanine label: a) those believed to bind (like tetrodotoxin) to the ion permeation channel itself, and b) those (see Catterall, J. Biol. Chem. 252:8669, 1977) believed to bind primarily to the regulatory subunits. Photoaffinity-labelled agents will be evaluated under voltage-clamp conditions in internally perfused crayfish giant axons to ensure no significant change in their specificity of action. Successful labels will then be utilized in Electrophorus electroplaque membranes to permit extraction and identification of functionally identified pore subunits (see Agnew et al., Proc. Natl. Acad. Sci. 75: 2606, 1978); a second proposal covering this phase of our project is now being prepared.